Killzone: Shadowfall for me, without doubt. It's just going to be a much deeper single-player experience, I think. I love how open some of the maps are that they've shown. Battlefield 4 will obviously be very multi-player centered, if you like that, though I'm willing to bet that Killzone will also have fun multi-player.
Of course, I don't care for Military shooters. Science fiction based games are my forte.